The present invention relates to a differential pressure type liquid level meter for use with a measuring receptacle, and more particularly to a differential pressure type liquid level meter by which a highly accurate liquid level measurement can be effected for various and large quantities of liquids.
It is well known in industrial measuring to use a differential pressure gauge to measure the quantity of such liquid as industrial solutions, oils and so on contained in a receptacle by detecting the pressure difference between a pressure exerted by the subject liquid and a reference pressure, for example the atmospheric pressure.
A known differential pressure type liquid level meter shown schematically in FIG. 5 comprises a measuring receptacle 10 having a measuring pressure cell 16 at the bottom thereof, a reference pressure cell 22, and a pressure difference detector 18 communicating with the pressure cells 16 and 22 by means of conduits 20 and 25. The measuring receptacle 10 is divided by a diaphragm 14 into two chambers, namely an upper chamber for containing a subject liquid 12 and a lower chamber as the pressure cell 16. On the other hand, the reference pressure cell 22 is closed by a diaphragm 23 exposed to the air. Between each diaphragm 14, 23 and the pressure difference detector 18, there is a liquid 16A, 22A sealed in each pressure cell 16, 22 and conduits 20, 25 and filling the same. Due to such pressure cells 16, 22, the pressure exerted on each diaphragm 14, 23 is transmitted to the pressure difference detector 18 through the sealed liquid 16A, 22A. In the pressure difference detector, the pressure difference between the pressures exerted on the respective diaphragms 14 and 23 can be detected.
In such a differential pressure type liquid-level meter, assuming that .rho. and h are representative of the density of the subject liquid 12 and the level of the liquid 12 to be measured in the measuring receptacle 10, respectively, the total pressure P applied to the diaphragm 14 is given by the following equation: EQU P=.rho..times.h+atmospheric pressure (1)
Therefore, the level h of the liquid 12 in the receptacle 10 can be obtained by subtracting the atmospheric pressure from the resulting total pressure P actually applied to the diaphragm 14. For this subtraction, the current atmospheric pressure is detected by the detector 18 through the pressure cell 22. Based on the pressures actually applied to the diaphragms 14 and 23, the pressure deference between the two is calculated in the detector 18. The level h of the liquid 12 in the receptacle 10 is obtained by dividing the resulting pressure difference by the density .rho. of the liquid 12.
There are well known in the art various manners of pressure difference detection such as mechanical equilibrium methods, displacement transforming methods, etc. Widely used are electrical displacement transforming detection methods which are known as strain gauge methods, capacitance methods, inductance methods, and so on.
There is, however, a problem in the conventional pressure difference type liquid-level meters, that temperature changes of the subject liquid and/or the ambient induce temperature changes of the sealed liquid (which is generally a silicone oil) 16A, 22A, in particular, the sealing liquid 16A in the pressure cell 16 adjacent to the measuring receptacle 10, resulting in a measurement error.
Specifically, letting the pressure change of the subject liquid 12 or the atmosphere which is exerted on the sealed liquid 16A or 22A through the diaphragm 14 or 23, and the volume change of the sealed liquids 16A, 22A, be .DELTA.P and .DELTA.V, respectively, when the temperature change of the sealed liquid 16A, 24A is infinitesimally .DELTA.T, the pressure change .DELTA.P is represented by the following equation: EQU .DELTA.P=.DELTA.V/.phi. (2)
wherein .phi. is a balancing factor [mm/kg] or a natural constant corresponding to an equivalent volume change which is produced when a unit pressure is exerted on the diaphragm. The balancing factor depends on the thickness and material of the diaphragm.
The volume change .DELTA.V of the sealed liquid 16A, 22A is represented by the following equation: EQU .DELTA.V=.alpha..times..DELTA.T.times.V (3)
wherein V is representative of the volume of the sealed liquid which is generally 3 to 15 cc and .alpha. is the thermal coefficient of the sealed liquid. The thermal expansion coefficient of the sealed liquid experimentally used was approximately 8.times.10.sup.-4 per degree C.
Therefore, the change .DELTA.P of pressure exerted on the diaphragm can be obtained from equations (2) and (3) in the following calculation: ##EQU1##
Taking for example water having a liquid column level h equal to 100mm, for the subject liquid to be measured, and assuming .DELTA.T to be 3.degree. C., a pressure change .DELTA.P of 1.2 mmH.sub.2 0 is produced in the sealed liquid. From this calculation, it is evident that the water having the liquid level of 100 mm is measured with an error of about 1.2%. Therefore, the temperature changes of the sealed liquid lead to noticeable measurement errors of liquid level.
Although it is said that an accurate measurement can be effected by controlling the liquid to be measured and/or the ambient to have a constant temperature, it is actually hard to control the temperature of a liquid to be measured because various chemical liquids are introduced into the measuring receptacle successively for measurement.